This invention relates to a channel impulse response estimator for use in an adaptive maximum likelihood sequence estimation receiver and, more particularly, to a channel impulse response estimator for use in an adaptive equalizer system for a digital mobile communication system.
In general, an adaptive maximum likelihood sequence estimation receiver of the type described is supplied with a received signal which is transmitted through a transmission path or a channel. The received signal has a waveform subject to a distortion which results from degradation of a channel characteristic or a characteristic in the transmission path and interruption in the transmission path. The adaptive maximum likelihood sequence estimation receiver serves to equalize the distortion of the waveform, as well known in the art.
An adaptive maximum likelihood sequence estimation receiver is disclosed in a book "Digital Communications" edited by Proakis, John G, New York, McGraw-Hill, 1983. The adaptive maximum likelihood sequence estimation receiver comprises a maximum likelihood sequence estimator and a channel impulse response estimator. The maximum likelihood sequence estimator carries out a sequence estimation operation on the received signal in response to a plurality of impulse response signals which collectively indicate a channel impulse response to produce an estimated sequence signal indicative of an estimated sequence after a lapse of a predetermined delay time. The received signal comprises a transmission preamble signal indicative of a transmission preamble and a data signal following the transmission preamble signal. The channel impulse response estimator carries out a response estimation operation on the received signal and the estimated sequence signal to produce the impulse response signals. In the manner which will later become clear, a conventional channel impulse response estimator includes a tap coefficient producer for producing a plurality of tap coefficient signals indicative of tap coefficients on the basis of a fixed step-size signal indicative of a fixed step size. As a result, the adaptive maximum likelihood sequence estimation receiver including the conventional channel impulse response estimator is disadvantageous in that it is difficult to apply to a communication system such as a digital mobile communication system having the channel characteristic which rapidly fluctuates.